WASHINGTON (AP) — The federal oversight board directed by President Barack Obama to scrutinize the government's secret surveillance system is hearing from civil liberties activists, a retired federal judge and a former Bush administration lawyer in the board's first public event since the spying operations were revealed in news reports.

They were among 16 experts set to testify Tuesday before the Privacy and Civil Liberties Oversight Board about the National Security Agency's surveillance. The board's five members include an Internet freedom advocate and two former Bush lawyers who helped expand the government's national security authority.

After former NSA systems analyst Edward Snowden began exposing the NSA's operations in June, Obama instructed the board to lead a "national conversation" about the secret programs. The board has been given several secret briefings by national security officials and it plans a comprehensive inquiry and a public report on the matter.

"Our primary focus will be on the programs themselves," the board's chairman, David Medine, told the Associated Press last month. "Based on what we've learned so far, further questions are warranted."

American Civil Liberties Union Deputy Legal Director Jameel Jaffer, who is expected to testify, warned the oversight board that the government's massive sweeps of cellphone and telephone call logs and other data on phone and Internet communications erode privacy protections guaranteed by the Fourth Amendment to the Constitution. The amendment protects against unreasonable search and seizure.

Snowden's disclosures revealed that the NSA collects phone "metadata" — records that omitted only the actual contents of conversations — from millions of Americans. A separate NSA surveillance program aimed solely at foreign terrorist suspects also sweeps up metadata about the Internet communications from smaller numbers of Americans, federal officials have acknowledged. Obama urged Americans not to worry about the secret programs because the contents of their communications are rarely targeted.

The president and the director of national intelligence "have been at pains to emphasize that the government is collecting metadata, not content," Jaffer said in advance remarks obtained by the AP. "But the suggestion that metadata is somehow beyond the reach of the Constitution is wrong. For Fourth Amendment purposes, the crucial question is not whether the government is collecting content or metadata but whether it is invading the reasonable expectation of privacy. Here, it clearly is."

Jaffer has urged stricter limits on government surveillance as well as new oversight mechanisms. One of the oversight board's members, James Dempsey, is a top official with the Center for Democracy and Technology, a civil liberties advocacy group.

Much of the NSA's surveillance is overseen by the secret Foreign Intelligence Surveillance Court, whose classified rulings also have been disclosed by several Snowden leaks. Testimony about the court was expected from former federal judge James Robertson, who served on the secret court. Robertson also ruled against the Bush administration in the landmark Hamdan vs. Rumsfeld case, which granted inmates at the U.S. naval prison at Guantanamo Bay, Cuba, the right to challenge their detentions. That ruling was upheld by the Supreme Court.

Steven Bradbury, a former top Bush administration lawyer who played a central role in national security decisions, also was to testify. Bradbury, who headed the Office of Legal Counsel at the Justice Department, wrote several legal memos backing the CIA's limited use of harsh interrogation techniques on terrorist suspects, including simulated drowning. But he also signed other memos that placed legal brakes on some of the Bush administration's expanded national security authority.

Two of the oversight board's members also worked as Bush administration lawyers. Elisebeth Collins Cook is a former Justice Department lawyer who drafted revised guidelines in 2008 that expanded the FBI's ability to conduct domestic intelligence investigations. The guidelines gave FBI agents involved in national security probes new authority to conduct physical surveillance without a court order and to interview people without identifying themselves as federal agents.

Oversight board member Rachel Brand is another former Bush Justice lawyer who urged Congress to give the FBI expanded authority to use what are called national security letters and administrative subpoenas to obtain information in terrorism investigations. The use of such tools is limited but can allow federal investigators to demand data without direct judicial approval.

The oversight board is appointed by the president but reports to Congress. David Cole, a professor of constitutional and national security law at Georgetown University, said the board faces high expectations.

"Their very existence may make government officials more careful about their surveillance programs because they will know that a board empowered and obligated to report on privacy and civil liberties abuses exists," Cole said.